Reverse electrodialysis cell and methods of use thereof

ABSTRACT

A method of generating electrical power or hydrogen from thermal energy is disclosed. The method includes separating, by a selectively permeable membrane, a first saline solution from a second saline solution, receiving, by the first saline solution and/or the second saline solution, thermal energy from a heat source, and mixing the first saline solution and the second saline solution in a controlled manner, capturing at least some salinity-gradient energy as electrical power as the salinity difference between the first saline solution and the second saline solution decreases. The method further includes transferring, by a heat pump, thermal energy from the first saline solution to the second saline solution, causing the salinity difference between the first saline solution and the second saline solution to increase.

FIELD

The present technology is generally related to systems and methods forgenerating electrical power and/or hydrogen from thermal energy.

BACKGROUND

Salinity gradient power is the energy created from the difference insalt concentration between two fluids, commonly fresh and salt waterthat naturally occurs, e.g., when a river flows into the sea. Reverseelectrodialysis (RED) can be used to retrieve energy from the salinitygradient, e.g., by passing a salt solution and fresh water through astack of alternating cation and anion exchange membranes. The chemicalpotential difference between the salt and fresh water generates avoltage over each membrane and the total potential of the system is thesum of the potential differences over all membranes. An open-loop REDbattery requires a continuous source of salt and fresh water to maintainthe salinity gradient. This constraint may limit practical locations ofcommercial-scale RED batteries. Furthermore, open-loop RED batteries aresusceptible to contamination from minerals, microbes, or other foreignobjects or material in the sources of water. Closed-loop RED cells donot require continuous sources of concentrated and dilute salinesolutions but do require ongoing regeneration of the salinity differencebetween the concentrated and dilute solutions which can be energyintensive and/or inefficient.

This document describes methods and systems that are directed toaddressing the problems described above, and/or other issues.

SUMMARY

The details of one or more aspects of the disclosure are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the techniques described in this disclosurewill be apparent from the description and drawings, and from the claims.

The present disclosure describes embodiments related to generatingelectrical power from thermal energy.

A reverse-electrodialysis system includes an anode, a cathode, and oneor more cells disposed between the anode and the cathode. At least oneof the one or more cells includes a first membrane configured to beselectively permeable to cations and a second membrane configured to beselectively permeable to anions, the second membrane spaced apart fromthe first membrane. The cell further includes a concentrated salinesolution disposed between the first membrane and the second membrane,the first and second membranes separating the concentrated salinesolution from a dilute saline solution such that the first membraneselectively allows cations to migrate toward the cathode and the secondmembrane selectively allows anions to migrate toward the anode, causinga voltage difference between the cathode and the anode. Thereverse-electrodialysis system further includes a heat source configuredto transfer thermal energy to the concentrated saline solution or thedilute saline solution and a regeneration system including a heat pump.The regeneration system is configured to receive the dilute salinesolution from the at least one of the one or more cells and remove (bythe heat pump) thermal energy from the dilute saline solution, causingthe dilute saline solution to precipitate a salt. The regenerationsystem is further configured to, after causing the dilute salinesolution to precipitate the salt, circulate the dilute saline solutionto the at least one of the one or more cells, introduce the precipitatedsalt into the concentrated saline solution, and cause the precipitatedsalt to dissolve in the concentrated saline solution.

Implementations of the disclosure may include one or more of thefollowing optional features. In some implementations, the regenerationsystem is further configured to transfer at least some of the thermalenergy removed from the dilute saline solution back to the dilute salinesolution after causing salt dissolved in the dilute saline solution toprecipitate. In some implementations, the regeneration system is furtherconfigured to transfer at least some of the thermal energy removed fromthe dilute saline solution to the concentrated saline solution, causingthe precipitated salt to dissolve in the concentrated saline solution.The heat source may be configured to transfer thermal energy to theconcentrated saline solution, causing the precipitated salt to dissolvein the concentrated saline solution. The concentrated saline solutionmay include an endothermic solution or an exothermic solution. Theconcentrated saline solution may include a substance having a solubilitywith a non-linear temperature dependence. In some examples, the firstmembrane and the second membrane include ion-exchange membranes. Thereverse-electrodialysis system may further include a control systemconfigured to coordinate the transfer of heat between one or more heatsources and the reverse-electrodialysis system based on one or moremeasurements of a state of the one or more heat sources or thereverse-electrodialysis system. The heat source includes one or more ofgeothermal heat, industrial waste heat, or solar heat.

In some embodiments, the reverse-electrodialysis system includes asecond cell. The second cell may include a third membrane configured tobe selectively permeable to cations and a fourth membrane configured tobe selectively permeable to anions, the fourth membrane spaced apartfrom the third membrane. The second cell may include a secondconcentrated saline solution disposed between the third membrane and thefourth membrane, the third and fourth membranes separating the secondconcentrated saline solution from a second dilute saline solution. Theconcentrated saline solution mat includes an endothermic solution, thesecond concentrated saline solution may include an exothermic solution,and the heat pump may be configured to transfer heat between theconcentrated saline solution and the second concentrated salinesolution.

In an embodiment, a method of generating electrical power from thermalenergy is disclosed. The method includes separating, by a selectivelypermeable membrane, a first saline solution from a second salinesolution. The method includes receiving, by the first saline solutionand/or the second saline solution, thermal energy from a heat source.The method includes mixing the first saline solution and the secondsaline solution in a controlled manner, capturing at least somesalinity-gradient energy as electrical power as the salinity differencebetween the first saline solution and the second saline solutiondecreases. The method includes transferring, by a heat pump, thermalenergy from the first saline solution to the second saline solution,causing the salinity difference between the first saline solution andthe second saline solution to increase.

Implementations of the disclosure may include one or more of thefollowing optional features. In some implementations, the method furtherincludes capturing the salinity-gradient energy using reverseelectrodialysis. In some implementations, the method further includescapturing the salinity-gradient energy pressure-retarded osmosis drivingan electrical generator. In some embodiments, each of the first salinesolution and the second saline solution circulate in a closed system.Transferring thermal energy from the first saline solution to the secondsaline solution may cause the first saline solution to precipitate asalt. The method may further include introducing the precipitated saltinto the second saline solution, causing the salinity difference betweenthe first saline solution and the second saline solution to increase.The method may further include using a portion of the generatedelectrical power to produce hydrogen gas through electrolysis. In someexamples, transferring thermal energy from the first saline solution tothe second saline solution includes transferring thermal energy from thefirst saline solution that is cooler than the second saline solution.

The method may further include coordinating the transfer of heat fromone or more heat sources to the first saline solution and/or the secondsaline solution based on one or more measurements of a state of the oneor more heat sources or the first saline solution and/or the secondsaline solution. The heat source may include one or more of geothermalheat, industrial waste heat, or solar heat.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated herein and constitutepart of this specification, are illustrative of particular embodimentsof the present disclosure and do not limit the scope of the presentdisclosure.

FIG. 1 illustrates an example reverse electrodialysis (RED) system.

FIG. 2 shows an example regeneration system.

FIG. 3 shows an example thermal optimization system.

FIG. 4 shows an example pressure-retarded osmosis (PRO) system.

FIG. 5 shows an example hydrogen-generating system.

FIG. 6 is a flowchart of a method for generating electrical power fromthermal energy.

FIG. 7 shows a block diagram of an example of internal hardware that maybe used to contain or implement program instructions according to anembodiment.

DETAILED DESCRIPTION

The following discussion omits or only briefly describes conventionalfeatures of the disclosed technology that are apparent to those skilledin the art. Reference to various embodiments does not limit the scope ofthe claims attached hereto. Additionally, any examples set forth in thisspecification are intended to be non-limiting and merely set forth someof the many possible embodiments for the appended claims. Further,particular features described herein can be used in combination withother described features in each of the various possible combinationsand permutations. A person of ordinary skill in the art would know howto use the instant invention, in combination with routine experiments,to achieve other outcomes not specifically disclosed in the examples orthe embodiments.

It is also to be understood that the terminology used in the descriptionis for the purpose of describing the particular versions or embodimentsonly and is not intended to limit the scope of the present disclosurewhich will be limited only by the appended claims. Unless otherwisespecifically defined herein, all terms are to be given their broadestpossible interpretation including meanings implied from thespecification as well as meanings understood by those skilled in the artand/or as defined in dictionaries, treatises, etc. Unless definedotherwise, all technical and scientific terms used herein have the samemeanings as commonly understood by one of ordinary skill in the art.Although any methods, equipment, and materials similar or equivalent tothose described herein can be used in the practice or testing ofembodiments of the present disclosure, the preferred methods, devices,and materials are now described. All references mentioned herein areincorporated by reference in their entirety.

As used in the specification and including the appended claims, thesingular forms “a,” “an,” and “the” include the plural, and reference toa particular numerical value includes at least that particular value,unless the context clearly dictates otherwise. Ranges may be expressedherein as from “about” or “approximately” one particular value and/or to“about” or “approximately” another particular value. When such a rangeis expressed, another embodiment includes from the one particular valueand/or to the other particular value. Similarly, when values areexpressed as approximations, by use of the antecedent “about,” it willbe understood that the particular value forms another embodiment. It isalso understood that all spatial references, such as, for example,horizontal, vertical, top, upper, lower, bottom, left and right, are forillustrative purposes only and can be varied within the scope of thedisclosure. For example, the references “upper” and “lower” are relativeand used only in the context to the other and are not necessarily“superior” and “inferior”. Generally, similar spatial references ofdifferent aspects or components indicate similar spatial orientationand/or positioning, i.e., that each “first end” is situated on ordirected towards the same end of the device.

The systems and methods described in this disclosure are generallydirected to efficiently extracting usable energy from the difference insalt concentration between two solutions via a precisely orchestratedand controlled mixing of the two solutions. The systems may be used todirectly generate electrical power or to generate hydrogen gas, whichcan be used as a fuel for generating mechanical (and/or electrical)power or to generate pressure and/or gravitational potential energy,either of which can be used to drive a turbine or perform other usefulwork. The systems encompass a range of sizes and power outputs. Someembodiments may be configured to generate power at the scale of a singleresidence or commercial building. In some examples, the systems includeindustrial power generation systems providing electrical power to aregional or national power grid. In some examples, the systems providehydrogen fuel, e.g., to power a fleet of vehicles, as well as (orinstead of) generating electrical power.

FIG. 1 shows an example system 100 for generating electricity from asalinity gradient. The example system 100 includes a reverseelectrodialysis (RED) battery 110. The RED battery 110 includes acathode 112 and an anode 114 separated by one or more cells 150, 150 a,150 b. Each cell 150 contains a salt solution 130, i.e., a liquidmixture of a solvent and a salt that has been dissolved into its anionicand cationic constituent components, such that the ionic constituentsare free to move with respect to each other. Each ion may have a singlecharge or may have multiple changes. In some examples, the solvent andsolute are water and sodium chloride (NaCl), respectively. Thedissociated ions of NaCl are Na⁺ and Cl⁻, each having a single charge.Other solvents and solutes may be used that also form a liquid mixtureincluding anions and cations that are free to move with respect to eachother. The salt solution may be exothermic or endothermic. That is, asthe solution is formed, the solution may either absorb heat, such aswhen potassium chlorate (KClO₃) or potassium nitrate (KNOB) is dissolvedin water, or give off heat, such as when calcium chloride (CaCl₂) isdissolved in water.

As shown in FIG. 1, the salt solution 130 is separated from a dilutesolution 140 (i.e., a solution having a lower solute concentration thanthe salt solution) by selectively permeable membranes 104, 104 a-d. Thesalt solution 130 is separated from the dilute solution 140 on one sideof the cell 150 by a cation-exchange membrane (104 a, 104 c) and on theother side of the cell 150 by an anion-exchange membrane (104 b, 104 d)which is spaced apart from the cation-exchange membrane (104 a, 104 c).The salt solution 130 is disposed in the space between thecation-exchange membrane (104 a, 104 c) and anion-exchange membrane (104b, 104 d). In the absence of these membranes 104, the salt solution 130would freely diffuse into the dilute solution 140, equalizing thesalinity of the two solutions. The selectively permeable membranes 104limit the ability of ionic constituents to freely diffuse. Instead, thecation-exchange membranes (104 a, 104 c) and anion-exchange membranes(104 b, 104 d) preferentially allow cationic constituents and anionicconstituents, respectively, to migrate, or move in opposite directions.The movement of negatively charged ions toward the anode 114 andpositively charged ions toward the cathode 112 causes an electricalpotential difference (voltage) across the cell 150. The total voltage ofthe battery 110 includes the voltages of each cell 150.

In some examples, the selectively permeable membranes 104 may be made oforganic or inorganic polymer with charged (ionic) side groups, such asion-exchange resins. The selectively permeable membranes 104 may also bemade of graphene, e.g., configured in thin sheets. The permeability ofthe membrane 104 may depend on configuration or other aspects of thegraphene sheets. In some examples, graphene sheets may be stretched orotherwise reconfigured to alter the permeability of the membrane 104.Ions may tend to accumulate near the membranes 104. This accumulationmay impede the process of generating power. To counteract thisaccumulation, the system 100 may apply an agitating or mixing force tothe salt solution 130 and/or the dilute solution 140, causing the ionsto be distributed more evenly (homogeneously) throughout the solution130, 140. In some examples, the system 100 applies sonic vibration toone or more solutions 130, 140 to enhance homogeneity of the solution130, 140. The system 100 may apply sonic vibration to areas of the cell150 where ions accumulate, e.g., near one or more membranes 104, toeffectively enhance homogeneity of one or more solution 130, 140.

As shown in FIG. 1, the electrodes (e.g., cathode 112, anode 114) aresurrounded by the dilute solution 140. Alternatively, the electrodes112, 114 may be surrounded by a rinse solution which is circulated in aclosed loop between the electrodes 112, 114 such that the rinse solutionis separate from the salt solution 130 and dilute solution 140. In suchan arrangement, the outer selectively-permeable membranes 104 (i.e., themembranes 104 closest to the respective electrodes 112, 114 of the REDbattery 110) are of the same type (e.g., both anion-exchange membranesor both cation-exchange membranes). For example, a cation-exchangemembrane 104 may separate the rinse solution surrounding the cathode 112from the salt solution 130, and a cation-exchange membrane 104 may alsoseparate the rinse solution surrounding the anode 114 from the dilutesolution 140. In this arrangement, the cations that migrate from thesalt solution 130 to the electrolyte surrounding the cathode 112 arerecirculated to the anode 114, where the cations may pass through thecation-exchange membrane 104 and into the dilute solution 140.Similarly, membranes 104 closest to the respective electrodes 112, 114may both be anion-exchange membranes, in which case anions arecirculated (in the rinse solution) from the anode 114 to the cathode112, where the anions pass through the anion-exchange membrane 104 intothe dilute solution 130. In either configuration, a reduction reactiontakes place at the cathode 112 and a balancing oxidation reaction takesplace at the anode 114. In some examples, the rinse solution includes asupporting electrolyte to enhance the reactions at the electrodes 112,114. Because both electrodes 112, 114 are surrounded by the sameelectrolyte (rinse solution) in this arrangement, the rinse solution mayform a resistive load between the electrodes 112, 114, through whichcurrent may flow, resulting in a reduction of power output of the REDbattery 110. In some examples, the rinse solution and/or associatedcirculation system may be configured to have a high resistance withrespect to the output load of the RED battery 110 or the internalresistance of cells 150 of the RED battery 110. For example, the rinsesolution circulation path may be configured to be relatively long.

The electrical current produced by the battery 110 is a function of therate of ion movement, and the rate of ion movement is a function ofseveral factors, including the salinity gradient (i.e., the salinitydifference between the salt solution 130 and the dilute solution 140)and the temperature of (at least) the salt solution 130, as well asaspects of the membranes 104. The temperature of the salt solution 130affects the rate at which ions in the salt solution 130 move toward (andacross) the membranes 104 due to the increased kinetic energy of theions at higher temperatures. According to the Nernst equation, the powerproduced is a function of the log of the salinity ration of the saltsolution 130 and dilute solution 140. As ions move from the saltsolution 130 to the dilute solution 140, however, the salinity of thedilute solution 140 increases and the salinity of the salt solution 130decreases. Thus the gradient between the “spent” salt solution 130 andthe “spent” dilute solution 140 decreases. To maintain electricalcurrent (and therefore, power output) of the battery 110, the salinitydifference may be continually regenerated by refreshing the spent saltsolution 130 and/or the spent dilute solution 140. To this end, thespent salt solution 130 and/or the spent dilute solution 140 may becirculated (e.g., in a closed loop) between the RED battery 110 and aregeneration system. Alternatively, the spent salt solution 130 and/orthe spent dilute solution 140 may be continually replenished, e.g., fromnatural sources such as rivers and oceans or bays.

FIG. 2 shows an example regeneration system 200. The exampleregeneration system 200 includes a salt removal subsystem 210. Theregeneration system 200 circulates spent dilute solution 140 in a closedloop from the RED battery 110 through the salt removal subsystem 210 andback to the RED battery 110 as refreshed dilute solution 140. In someexamples, the salt removal subsystem 210 evaporates and then condensesthe solvent of the dilute solution 140 then circulates the condensedsolvent back to the RED battery 110 as refreshed dilute solution 140.The system may evaporate solvent until the remaining dilute solution 140is close to, or even less than, the solubility limit. The remainingdilute solution 140 may then be reintroduced into the spent saltsolution 130 to refresh the salt solution 130. In some examples, thesalt removal subsystem 210 evaporates and then condenses the solvent ofthe salt solution 130 and then circulates the condensed solvent back tothe RED battery 110 as refreshed dilute solution 140. In a closed-loopRED battery 110, the salt removal subsystem 210 may evaporate, thencondense both the salt solution 130 and the dilute solution 140,circulate the condensed solvent from both solutions back to the REDbattery 110 as refreshed dilute solution 140, and circulate theremaining solution back to the RED battery 110 as refreshed(concentrated) salt solution 130. In some examples, the salt removalsubsystem 210 decreases the salinity of the dilute solution 140 throughthe process of salt precipitation, rather than (or in addition to)evaporation, to regenerate the salinity difference between the saltsolution 130 and dilute solution 140 in the RED battery 110. Othermethods of regenerating the salinity gradient include freezing the spentdilute solution 140, e.g., through Eutectic Chilled Crystallization(ECC), or using microfiltration and/or membrane separation. In someexamples, multiple methods are advantageously combined. For example, aprecipitation or freezing stage may be enhanced by a subsequent membranefiltration stage in order to optimize the total energy required toseparate the salt from the spent dilute solution 140.

As shown in FIG. 2, the example salt removal subsystem 210 removes saltfrom the spent dilute solution 140 by causing salt to precipitate.Generally, the ability of a solvent to dissolve a solute increases withincreased temperature. Conversely, lowering the temperature of asolution below the temperature known as the saturation point (thetemperature at which the solution is at its maximum salinity) willgenerally cause the solute to precipitate. The example salt removalsubsystem 210 includes a heat transfer device 216 configured to cool thespent dilute solution 140 to a temperature below the saturation point.If the spent dilute solution 140 is an exothermic solution, the dilutesolution 140 will further cool as the salt precipitates. In someexamples, the heat transfer device 216 also heats the refreshed dilutesolution 140, e.g., back to the temperature of the spent dilute solution140 prior to cooling. That is, the heat transfer device 216 may transfersome or all of the thermal energy removed from the spent dilute solution140 back to the refreshed dilute solution 140, as shown by the arrowassociated with the heat transfer device 216 of FIG. 2. In this way, thetemperature of the refreshed dilute solution 140 entering the REDbattery 110 is substantially the same as the temperature of the spentdilute solution 140 leaving the RED cell 150.

In the example salt removal subsystem 210, the precipitated salt 212settles to the bottom of the salt removal subsystem 210, e.g., in adense solid form. In some examples, the salt removal subsystem 210includes a conveying device 230 configured to transport the precipitatedsalt 212 away from the salt removal subsystem 210. The conveying device230 may be a belt, pump, Archimedes screw, or other device or systemconfigured to physically transport the precipitated salt 212 away fromthe salt removal subsystem 210. For example, if the salt is in a solidform, the conveying device 230 may be a mechanical system capable oftransporting solid material. In some examples, the removed salt 212 isconveyed to a salt replenishment subsystem 220 where the salt isreintroduced (e.g., redissolved) into the spent salt solution 130, thusrefreshing the spent salt solution 130. In a similar manner to how theregeneration system 200 circulates dilute solution 140, the regenerationsystem 200 may also circulate spent salt solution 130 in a closed loopfrom the RED battery 110 through the salt replenishment subsystem 220and back to the RED battery 110 as refreshed salt solution 130. The saltreplenishment subsystem 220 may increase the salinity of the saltsolution 130 through the process of redissolving the salt removed by thesalt removal subsystem 210, thus regenerating the salinity differencebetween the salt solution 130 and dilute solution 140 in the RED battery110.

As described above, the ability of solvent to dissolve a solutegenerally increases with increased temperature. Thus, highertemperatures of the salt solution 130 allow for higher levels ofsalinity and the accompanying greater differences between the salinityof the salt solution 130 and the dilute solution 140. A solubility curveis a plot of the amount of a solute that a specific amount of solventcan dissolve as a function of temperature. In some examples, asolubility curve associated with a solution is linear. That is, theamount of solute that the solvent can dissolve may change linearly withtemperature change over a wide range of temperatures (e.g., the entirerange that the solvent is a liquid). In some examples, the amount ofsolute that the solvent can dissolve changes non-linearly withtemperature change. In these cases, the amount of solute that thesolvent can dissolve may increase by, e.g., a factor of five or more,even within a narrow range of temperatures. The system 100 may beconfigured to operate the RED battery 110 within a temperature rangewhere the salinity of the salt solution 130 is high. Dissolvingadditional salt may require transferring additional heat to the saltsolution 130. Furthermore, the system 100 may maintain the temperatureof the RED battery 110 at a point above the solubility point to providea “safety margin,” to avoid unwanted precipitation if the salt solution130 cools below the solubility point.

The salt replenishment subsystem 220 may receive thermal energy from oneor more heat sources configured to increase the temperature of the saltsolution 130, e.g., to allow additional salt to dissolve. For example,the salt replenishment subsystem 220 may receive waste heat from theheat transfer device 216 of the salt removal subsystem 210. The saltreplenishment subsystem 220 may also be configured to receive thermalenergy from other heat sources as well, as shown by the arrow associatedwith the salt replenishment subsystem 220 of FIG. 2. Examples of heatsources include, but are not limited to, geothermal heat, industrialwaste heat, solar heat, combustion heat, vapor-compression-cycle wasteheat (e.g., from a heat pump), chemical reaction heat, or other forms ofheat that are not readily or efficiently converted to usable formsthrough conventional means, such as a by driving a turbine.

A thermal optimization system may be used to optimize the use of thermalenergy with the power generation system 100 described herein. Thermaloptimization systems are further described in U.S. Pat. No. 11,067,317,which is hereby incorporated by reference in its entirety. The thermaloptimization system may transfer thermal energy from one or more heatsources to one or more heat sinks. Examples of heat sinks include theinterior of living or office spaces during cooler seasons of the year,heated swimming pools, saunas, and steam rooms. In these examples, thesystem may be configured to regulate a temperature by modulating thetransfer of thermal energy to a heat sink. For example, the thermaloptimization system may monitor the temperature of the heated spacesand/or heated water and modulate the transfer of heat usingprocessor-based logic, such as (but not limited to) running one or morePID feedback loops and/or expert systems. During hotter seasons, theinterior spaces may be heat sources. In this case, the processor-basedregulation system may modulate the transfer of thermal energy away fromthese spaces to regulate the temperature.

FIG. 3 shows an example thermal optimization system 300. The exampleoptimization system 300 includes one or more heat sources 305, one ormore heat sinks 310, and one or more RED batteries 110 as describedabove. In some examples, the power generation system 100 includes apressure-retarded osmosis (PRO) system (discussed in more detail below),or other systems for generating electricity from a salinity gradientinstead of (or in addition to) a RED battery 110. The optimizationsystem 300 may include one or more pumps 320 configured to transfer heatfrom one location to another. In some examples, the optimization system300 uses vapor-compression refrigeration to transfer heat from a heatsource to a heat sink. That is, the optimization system 300 may compressa refrigerant to transfer thermal energy from the refrigerant to a heatsink (e.g., by a heat exchanger configured for the purpose of absorbingand distributing thermal energy). The optimization system 300 may thentransfer the compressed refrigerant (e.g., by pumping the compressedrefrigerant via piping configured for the purpose) to a heat source andallow the refrigerant to expand, absorbing thermal energy from the heatsource (e.g., via a heat exchanger configured for the purpose ofproviding thermal energy from the heat source). The vapor-compressionrefrigeration system may include a reversing valve or other controllabledevice that causes the transfer of thermal energy to change direction.The optimization system 300 may control the reversing value to changethe direction of heat transfer, e.g., to cool an interior space duringthe day, when ambient temperatures are relatively high, then reverse theheat flow to heat the interior space at night as ambient temperaturesdrop.

Operation of the thermal optimization system 300 may be coordinated by acontrol system 350 having a processor, e.g., as described below withrespect to FIG. 7. The processor executes instructions causing thecontrol system 350 to coordinate the transfer of heat between heatsources 305, heat sinks 310, and RED batteries 110 (or PRO systems),e.g., based on prevailing conditions. The control system 350 may alsocoordinate the transfer of heat within the power generation system 100,e.g., between subsystems such as the salt removal subsystem 210 and thesalt replenishment subsystem 220. The control system 350 may transmit orreceive signals 352, 352 a-c to or from the heat sources 305, heat sinks310, and RED battery 110 (or PRO system). The control system 350 mayreceive signals 352 indicating one or more conditions or states of,e.g., the heat sources 305, heat sinks 310, and RED battery 110 (or PROsystem). For example, the signals 352 may indicate measured quantitiessuch as temperatures and/or pressures, or the signals may indicate userinputs, such as target temperatures for heated interior spaces.Temperatures associated with power generation system 100 may includetemperatures of the salt solution 130 and dilute solution 140 in the REDbattery, in the salt removal subsystem 210, and/or in the saltreplenishment subsystem 220, respectively.

A control system 350 may be used to transmit control signals, e.g., tocontrol the speed of a compressor and/or pump, a direction of areversing valve, the operating speed of the salt conveying/transferdevice 230, etc. to achieve the indicated target temperatures and/orpower outputs. For example, the control system 350 may adjust thetemperature of the salt solution to be at or near its solubility limit.In this way, the control system 350 may efficiently control and modulatethe transfer of heat between several heat sources 305 and heat sinks 310simultaneously, based on prevailing conditions and user settings, whileoptimizing the output of the power generation system 100. Furthermore,the control system 350 may affect the level of vapor-compression-cyclewaste heat generated by one or more heat pumps 320 and transfer thewaste heat to one or more heat sinks and/or to the RED battery 110, thusefficiently recapturing its own waste heat for power generation or otherpurposes. In this way, the optimization system 300 may transfer heatfrom any or all of a variety of heat sources, under a variety of dynamicconditions (e.g., as conditions change throughout the year or throughoutthe day) and/or based on demands of the RED battery 110. Furthermore,the control system 350 may configure the power generation system 100 tostore excess energy. For example, when demand for electrical energy islow, the control system 350 may configure the RED battery 110 to producea portion of its energy output as hydrogen gas to be used as fuel at alater time, rather than as electrical energy to be used at the time ofgeneration. Furthermore, in the case of a PRO system, the control system350 may configure the rate at which pressure is converted toelectricity, e.g., by controlling the rate of flow through a turbine. Inthis way, the control system 350 may retain some energy in the form of,e.g., gravitational potential energy when demand for electrical power islow and convert greater amounts of the gravitational potential energy toelectrical energy when demand for electrical power is high.

In some embodiments, the system 100 includes a PRO system instead of (orin addition to) the RED battery 110 described above. FIG. 4 shows anexample PRO system 120. The example PRO system 120 also includes a saltsolution 130 separated from a dilute solution 140 by a selectivelypermeable membrane 104 c, similar to the RED battery 110 describedabove. However, rather than directly generating electrical power fromthe salinity difference between the salt solution 130 and the dilutesolution 140, the PRO system generates pressure, which may take the formof gravitational potential energy. Therefore, the PRO system may notinclude electrodes. The selectively permeable membranes of the PROsystem may be configured to preferentially allow solvent, rather thansolute, to pass through the membrane, e.g., from the dilute solution 140to the salt solution 130 so as to decrease the salinity differencebetween the solutions. As a result, the pressure and/or the averageheight of the solvent in the salt solution 130 may increase over time assolvent migrates across the membrane 104 c into the salt solution 130.The system 100 may convert the increased pressure and/or gravitationalpotential energy of the salt solution 130 into a more usable form ofpower, such as electrical energy, e.g., by directing the (raised) saltsolution 130 through a turbine, paddlewheel, or other suitable mechanismfor producing electricity. The system 100 may then circulate the spentsalt solution 130 through a solvent removal system similar to the saltremoval subsystem 210 described above. For example, the solvent recoverysystem may remove the excess solvent through evaporation (optionally ina partial vacuum to reduce the boiling point) and subsequentcondensation. The solvent recovery system may then circulate thecondensed solvent back to the PRO system 120 as refreshed or “make up”dilute solution 140 and circulate the refreshed salt solution (afterexcess solvent is removed) back to the PRO system 120 as refreshed saltsolution 130. Alternatively (or additionally), the solvent recoverysystem may cause the solute to precipitate out of the spent saltsolution 130 before circulating the spent salt solution back to the PROsystem 120 as “make up” dilute solution 140 and introducing theprecipitated salt back into the salt solution 130, e.g., via theconveying device 230 of FIG. 2. As with the RED battery 110 describedabove, the rate of power generated by the PRO system 120 is a functionat least of the salinity difference between the salt solution 130 andthe dilute solution 140 and the temperature of at least the saltsolution 130.

As shown in FIG. 1, the salt solution 130 is separated from the dilutesolution 140 on one side of the cell 150 by a cation-exchange membrane(104 a, 104 c) and on the other side of the cell 150 by ananion-exchange membrane (104 b, 104 d). Alternatively, the salt solution130 and the dilute solution 140 may be separated by a single selectivelypermeable membrane 140 (e.g., a cation-exchange membrane or ananion-exchange membrane). The selective movement of ions across thesingle membrane 140 causes an electrical potential difference (voltage)across the membrane 140. Similar to the embodiment of FIG. 1, electricalcurrent produced by a single-membrane embodiment of a RED cell 150(e.g., a flow pump) is also a function of the rate of ion movement, andthe rate of ion movement is a function of several factors, including thesalinity gradient between the salt solution 130 and the dilute solution140, the temperature of (at least) the salt solution 130 (due to theincreased kinetic energy of the ions at higher temperatures), andaspects of the single membrane 104.

In some embodiments, the system includes a first RED battery 110 a,configured to use an exothermic salt solution 130 a, and a second REDbattery 110 b, configured to use an endothermic salt solution 130 b. Thesystem may transfer heat generated by dissolving the solute in theexothermic salt solution 130 a to the endothermic solution 130 b toreplace heat absorbed while dissolving the solute.

In some embodiments, a portion of the generated electrical power is usedto produce hydrogen gas, e.g., by decomposing water throughelectrolysis. For example, when the dilute solution is water, apotential difference of 1.23 volts may be applied to the water to splitthe water into hydrogen and oxygen. Either the salt solution or thedilute solution (or both) may be decomposed through electrolysis. FIG. 5shows an example RED battery 110 configured to generate hydrogen. Theliberated hydrogen and oxygen gases may “bubble” to the surface near therespective cathode 112 and anode 114 of the RED battery 110. The system100 may separate the oxygen and hydrogen gases (e.g., by physicallyseparating the cathode 112 and anode 114), capture and store thehydrogen (e.g., capture the hydrogen gas as it bubbles to the surfacenear the cathode), and then transport the hydrogen, by appropriate meansand to appropriate locations, for use as a fuel. In these cases, thesystem 100 must replenish or “make up” the solvent lost to electrolysis,e.g., from a stream or other source of fresh water. The RED battery 110may be configured such that electrolysis naturally occurs. That is, theRED battery 110 may be configured to generate sufficient potentialdifference to cause electrolysis of its solvent. In some examples, theregeneration system uses electrolysis to refresh the salinity of thespent salt solution 130 as make up fresh water is circulated to the REDbattery 110 (or PRO system) as refreshed dilute solution 140. In someexamples, the system includes a separate reservoir of water forgenerating hydrogen gas through electrolysis (e.g., rather thanelectrolyzing the solvent of the RED battery or PRO system). Theseparate water reservoir may have its own “make up” source while the REDbattery or PRO system remains closed loop. In these embodiments, thereservoir of water may also act as a heat reservoir or play other rolein the heat optimization process.

FIG. 6 shows a flowchart 600 of an example method for generatingelectrical power from thermal energy. At step 602, the example methodincludes separating, by a selectively permeable membrane 104, a firstsaline solution 130 from a second saline solution 140. The selectivelypermeable membrane 104 may be configured to provide a controlled mixingof the first saline solution 130 and second saline solution 140 so as tocapture the salinity-gradient energy in a more useful form as the mixingoccurs. In some examples, the selectively permeable membrane 104 isconfigured to preferentially allow solvent of the first saline solution130 to pass through the membrane and into the separate second salinesolution 140, e.g., as in a PRO system. In some examples, theselectively permeable membrane 104 is configured to preferentially alloweither anions or cations of the first saline solution 130 to passthrough the membrane and into the separate second saline solution 140,e.g., as in a RED battery 110.

At step 604 the example method includes receiving thermal energy from aheat source. The power generated by the RED battery 110 (or PRO system)is a function of temperature. The received thermal energy may allow theRED battery 110 to continue to operate (e.g., produce electricity). Insome examples, the control system 350 is configured to modulate theamount of thermal energy received and to configure which heat sources305 provide the thermal energy. In some examples, the control system 350is configured to transfer waste heat from one or more heat pumps 320 tothe RED battery 110. In some embodiments, the power generation system100 provides some or all of the power to operate one or more heat pumps320. As a prophetic example, a RED battery 110 may have an efficiency ofabout 30% (i.e., 30% of the thermal energy transferred to the REDbattery 110 is converted to electricity or other usable form of energy).A heat pump 320 may have a coefficient of performance (COP) between 3and 4 (i.e., the heat pump 320 may require 1 KW of power to take up 2-3KW of power from a heat source and transfer 3-4 KW to a heat sink (thesum of the input power and the thermal power taken up from the heatsource). For example, a heat pump 320 with a COP of 4 may require 1 KWof power to transfer a total of 4 KW of heat to the RED battery 110. Theheat pump 320 may transfer heat from a low-grade or “waste” heat source,e.g., a source that is not readily converted to a useful form of energy,such as a heat source less than 300 degrees C. In some predictedexamples, the heat source may be the result of an industrial processwhich would otherwise simply output the waste heat to the environment.With an efficiency of 30%, the RED battery 110 may produce 1.2 KW ofelectrical power from the 4 KW of transferred heat. In this predictiveexample, 1 KW of the electrical power may be used to power the heat pump320, leaving 200 W of electrical power for other purposes. Thus, in thispredictive example, the combined system 100 of the RED battery 110 andheat pump 320 produces a net output of 200 W of electrical power with nonet input of power other than the 3 KW of “waste” heat. In cases wherethe waste heat is the result of an industrial process, it is predictedthat the combined system 110 of the RED battery 110 and heat pump 320produces a net output of 200 W while simultaneously providing thebenefit of cooling the waste heat by 3 KW before outputting it to theenvironment. The anticipated net efficiency of the combined system 110may be further amplified with improvements to the efficiency of the REDbattery 110.

Furthermore, the power generation system 100 may enhance the effectivecoefficient of performance (COP) of a heat pump 320, e.g., a heat pumpused to heat or cool an inhabited space, by capturing some waste energyproduced by one or more heat pumps and converting the waste energy intoelectrical energy to power the heat pump 320. For example, a heat pumpwith a heating COP of 3 may require 1.5 KW of power to pump 3 KW of heatfrom a source to a sink. If the heat sink does not require the full 4.5KW of power (3 KW of pumped heat plus up to 1.5 KW of waste heat), thecontrol system 350 may configure the optimization system 300 to transfersome or all of the waste heat to the RED battery 110 for conversion topower for the heat pump 320, increasing the effective COP of the heatpump 320. Furthermore, the control system 350 may configure the powergeneration system 100 to convert some amount to waste energy into a formwhich can be stored for later use, e.g., if the instantaneous demand forelectrical power is greater than the amount of electrical power that canbe produced. For example, a PRO system may retain waste energy in theform of unreleased pressure and/or gravitational potential energy, to bereleased at a future time, e.g., when demand for electrical energy isgreater. Similarly, a RED battery 110 may produce hydrogen gas, to beused as fuel at a future time, in lieu of a producing some amount ofelectrical energy. Thus, waste heat from the heat pump 320 may beflexibly captured and released to further increase the effective COP ofthe heat pump.

At step 606 the example method includes mixing the first saline solution130 and the second saline solution 140 in a controlled manner. At step608, the example method includes capturing at least somesalinity-gradient energy as electrical power. As described above, theRED battery 110 or PRO system may be configured such that as thesolutions (130, 140) mix, the salinity-gradient energy is converted intoa more useful form. At step 610, the example method includestransferring, by a heat pump 320, thermal energy from the first salinesolution to the second saline solution. At step 612 the example methodincludes causing the salinity difference between the first salinesolution and the second saline solution to increase. As described above,the heat pump 320 may cool the spent dilute solution 140, causing thesalt to precipitate from the dilute solution 140, thus refreshing thedilute solution. The heat pump may transfer the heat from the spentdilute solution 140 to the spent salt solution 130, enhancing theprocess of dissolving salt introduced into the salt solution 130.Alternatively (or in addition), the heat pump 320 may heat the spentsalt solution 140, causing the salt solution 140 to evaporate, thusrefreshing the salt solution. The evaporated solvent may be condensed(e.g., cooled by the heat pump) as the solvent vapor is circulated backto the RED battery 110 as refreshed dilute solution.

It should be understood that various aspects disclosed herein may becombined in different combinations than the combinations specificallypresented in the description and accompanying drawings. It should alsobe understood that, depending on the example, certain acts or events ofany of the processes or methods described herein may be performed in adifferent sequence, may be added, merged, or left out altogether (e.g.,all described acts or events may not be necessary to carry out thetechniques). In addition, while certain aspects of this disclosure aredescribed as being performed by a single module or unit for purposes ofclarity, it should be understood that the techniques of this disclosuremay be performed by a combination of units or modules associated with,for example, a RED battery, a PRO system, a hydrogen generationsubsystem, a salt precipitation subsystem, an evaporation subsystem,etc.

In one or more examples, the described techniques may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored as one or more instructions orcode on a computer-readable medium and executed by a hardware-basedprocessing unit. Computer-readable media may include non-transitorycomputer-readable media, which corresponds to a tangible medium such asdata storage media (e.g., RAM, ROM, EEPROM, flash memory, or any othermedium that can be used to store desired program code in the form ofinstructions or data structures and that can be accessed by a computer).

Instructions may be executed by one or more processors, such as one ormore digital signal processors (DSPs), general purpose microprocessors,application specific integrated circuits (ASICs), field programmablelogic arrays (FPGAs), or other equivalent integrated or discrete logiccircuitry. Accordingly, the term “processor” as used herein may refer toany of the foregoing structure or any other physical structure suitablefor implementation of the described techniques. Also, the techniquescould be fully implemented in one or more circuits or logic elements.

FIG. 7 illustrates example hardware that may be used to contain orimplement program instructions. A bus 710 serves as the main informationhighway interconnecting the other illustrated components of thehardware. Central Processing Unit (CPU) 705 is the central processingunit of the system, performing calculations and logic operationsrequired to execute a program. CPU 705, alone or in conjunction with oneor more of the other elements disclosed in FIG. 7, is an example of aprocessor as such term is used within this disclosure. Read only memory(ROM) and random-access memory (RAM) constitute examples ofnon-transitory computer-readable storage media 720, memory devices ordata stores as such terms are used within this disclosure.

Program instructions, software or interactive modules for providing theinterface and performing any querying or analysis associated with one ormore data sets may be stored in the memory device 720. Optionally, theprogram instructions may be stored on a tangible, non-transitorycomputer-readable medium such as a compact disk, a digital disk, flashmemory, a memory card, a universal serial bus (USB) drive, an opticaldisc storage medium and/or other recording medium.

An optional display interface 730 may permit information from the bus710 to be displayed on the display 735 in audio, visual, graphic oralphanumeric format. Communication with external devices may occur usingvarious communication ports 740. A communication port 740 may beattached to a communications network, such as the Internet or anintranet.

The hardware may also include an interface 745 which allows for receiptof data from input devices such as a keypad 750 or other input device755 such as a touch screen, a remote control, a pointing device, a videoinput device and/or an audio input device.

It will be appreciated that the various above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications orcombinations of systems and applications. Also, that various presentlyunforeseen or unanticipated alternatives, modifications, variations orimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompassed by the following claims.

What is claimed is:
 1. A reverse electrodialysis system comprising: ananode; a cathode; one or more cells disposed between the anode and thecathode, at least one of the one or more cells comprising: a firstmembrane configured to be selectively permeable to cations; a secondmembrane configured to be selectively permeable to anions, the secondmembrane spaced apart from the first membrane; and a concentrated salinesolution disposed between the first membrane and the second membrane,the first and second membranes separating the concentrated salinesolution from a dilute saline solution such that the first membraneselectively allows cations to migrate toward the cathode and the secondmembrane selectively allows anions to migrate toward the anode, causinga voltage difference between the cathode and the anode; a thermaloptimization system configured to transfer thermal energy to theconcentrated saline solution or the dilute saline solution; and aregeneration system comprising a heat pump, the regeneration systemconfigured to: receive the dilute saline solution from the at least oneof the one or more cells; remove, by the heat pump, thermal energy fromthe dilute saline solution, causing the dilute saline solution toprecipitate a salt; after causing the dilute saline solution toprecipitate the salt, circulate the dilute saline solution to the atleast one of the one or more cells; introduce the precipitated salt intothe concentrated saline solution; cause the precipitated salt todissolve in the concentrated saline solution; and transfer at least someof the thermal energy removed from the dilute saline solution back tothe dilute saline solution after causing salt dissolved in the dilutesaline solution to precipitate.
 2. The reverse electrodialysis system ofclaim 1, wherein the regeneration system is further configured totransfer at least some of the thermal energy removed from the dilutesaline solution to the concentrated saline solution, causing theprecipitated salt to dissolve in the concentrated saline solution. 3.The reverse electrodialysis system of claim 1, wherein the heat sourceis configured to transfer thermal energy to the concentrated salinesolution, causing the precipitated salt to dissolve in the concentratedsaline solution.
 4. The reverse electrodialysis system of claim 1,wherein the concentrated saline solution comprises an endothermicsolution or an exothermic solution.
 5. The reverse electrodialysissystem of claim 1, wherein the concentrated saline solution comprises asubstance having a solubility with a non-linear temperature dependence.6. The reverse electrodialysis system of claim 1, wherein the firstmembrane and the second membrane comprise ion-exchange membranes.
 7. Thereverse electrodialysis system of claim 1, further comprising: a secondcell, the second cell comprising: a third membrane configured to beselectively permeable to cations; a fourth membrane configured to beselectively permeable to anions, the fourth membrane spaced apart fromthe third membrane; and a second concentrated saline solution disposedbetween the third membrane and the fourth membrane, the third and fourthmembranes separating the second concentrated saline solution from asecond dilute saline solution, wherein: the concentrated saline solutioncomprises an endothermic solution; the second concentrated salinesolution comprises an exothermic solution; and the heat pump isconfigured to transfer heat between the concentrated saline solution andthe second concentrated saline solution.
 8. The reverse electrodialysissystem of claim 1, wherein the thermal optimization system comprising acontrol system, the control system configured to, by a processor,coordinate the transfer of heat between one or more heat sources and thereverse electrodialysis system based on one or more measurements of astate of the one or more heat sources or the reverse electrodialysissystem.
 9. The reverse electrodialysis system of claim 8, wherein theheat source comprises one or more of geothermal heat, industrial wasteheat, or solar heat.